A fluid circuit is generally fitted with a filter that serves to filter the fluid, i.e. to remove impurities and residues therefrom that might otherwise damage the equipment in which the fluid flows. By way of example the fluid may be a lubricant such as oil, or it may be a fuel.
In use, by recovering such impurities and residues, the filter clogs up progressively, and ends up preventing fluid from passing through the filter. In order to avoid that situation, the branch of the fluid circuit having the filter mounted therein includes a bypass circuit in parallel. This configuration is shown in FIG. 1, which shows the state of the art. In the description below, the terms “upstream” and “downstream” are defined relative to the normal flow direction of the fluid in the circuit. The filter 20 is mounted in a main branch 10 of a fluid circuit. Upstream from the filter, the main branch 10 splits into two branches, a first branch 12 extending the main branch 10 and having the filter 20 mounted therein, and a second branch 14 having a check valve device 40 mounted therein (a “bypass” device). Downstream from the bypass device 40, the second branch 14 rejoins the first branch 12 downstream from the filter 20 so as to reform the main branch 10, such that the bypass device 40 is mounted in parallel with the filter 20. Consequently, the fluid pressure upstream from the bypass device 40 is substantially equal to the pressure P upstream from the filter 20. In FIG. 1, the fluid flows from left to right, as represented by arrow F.
When the filter is not clogged with impurities, i.e. at the beginning of its operation, the pressure Pf immediately downstream from the filter 20 (in the first branch 12) is substantially equal to the pressure P upstream from the filter 20. Similarly, the pressure Pdc immediately downstream from the bypass device 40 (in the second branch 14) is substantially equal to the pressure P upstream from the bypass device 40. All of the fluid flows in the branch 12, and the fluid in the upstream portion of the branch 14 is blocked by the bypass device 40. This bypass device 40 behaves like a valve that allows fluid to pass only when the pressure difference ΔP=P−Pdc from upstream to downstream across the bypass device 40 is greater than a critical value ΔPC. Such a bypass device 40 is known in the prior art. For example it may be a device comprising a ball valve, operating in the manner recalled briefly above.
A device with a ball valve comprises a container containing a ball and a spring. When the ball valve device is in its closed position, the ball is urged by the spring to block the upstream end of the container and thereby prevents the fluid in the portion of the branch situated upstream from the container from penetrating therein. When the pressure of the fluid upstream from the container increases relative to the pressure of the fluid downstream from the container, the force exerted by said fluid by the ball, and thus against the spring, increases in proportion. At a certain critical value ΔPC for the pressure difference between upstream and downstream of the ball valve device, the reaction force from the spring is exactly counterbalanced by the force generated by the pressure ΔPC on the ball. When this pressure exceeds the critical value ΔPC, the ball is pushed downstream and thus allows the fluid to pass through the ball valve device, which is then in its open position.
It will be understood that as the filter 20 becomes clogged as a result of impurities accumulating thereon, the upstream to downstream pressure difference ΔP=P−Pf across the filter 20 increases. The pressure difference across the bypass device 40 also increases since this difference is substantially equal to the pressure difference across the filter, since the first branch 12 including the filter 20 and the second branch 14 including the bypass device are in parallel. The bypass device 40 is calibrated in such a manner as to switch to its open position when the filter 20 becomes completely clogged. Thus, the critical value ΔPC is referred to as the clogging value (or the “clogging pressure difference”).
Once the filter 20 is completely clogged, the fluid thus passes via the second branch 14 of the circuit, through the bypass device 40, and is therefore no longer filtered. The fluid therefore continues to reach the downstream mechanism for which it is intended (e.g. an engine if the fluid is a fuel, a ball bearing or any other moving part if the fluid is an oil), but it does so filled with impurities. The impurities run the risk of damaging the mechanism, and in practice it is therefore essential to replace the filter 20 before it becomes completely clogged.
That is why the filter 20 is used in association with a pressure detector that informs the user (e.g. the pilot) that the filter 20 is about to become clogged and needs to be replaced quickly. FIG. 1, showing the prior art, shows one such detector 60. The detector 60 is mounted in a third branch 16 that is connected to the first branch 12 on either side of the filter 20. The detector 60 is a device that is known in the prior art (e.g. it is of the pressure contact type) and its structure is not described in detail here. It operates on the following principle: when the pressure difference ΔPm across the detector 60 reaches a threshold value ΔPS′ the pressure contact sends a signal (typically an electrical signal) to the user to inform the user that the pressure difference ΔPm across the detector 60 has crossed the threshold value ΔPS. The threshold value ΔPS is selected to be less than the clogging critical value ΔPC (e.g. 90% of said critical value). Thus, when the signal coming from the detector 60 is received by the user, the user knows that the filter 20 needs to be replaced soon, and sufficient time remains for the replacement to take place before the filter 20 becomes completely clogged. This information is passed on to the maintenance crew who perform the change in good time during a subsequent maintenance operation.
When the filter 20 is still not completely clogged, merely replacing the filter suffices prior to putting the circuit back into operation. However, it can happen that the maintenance crew performs this maintenance operation late, possibly after the filter 20 has become completely clogged. Under such circumstances, the filter 20 will have been bypassed and fluid will have continued to flow in the circuit, taking impurities through the bypass device 40 into the entire circuit downstream from the filter 20. Under such circumstances, it is essential for the circuit to be inspected exhaustively and cleaned, which operations are much more burdensome than replacing a filter. When the maintenance crew are performing a maintenance operation, it is therefore necessary for them to know whether or not the filter has already become clogged and has been bypassed.
That is why the portion of the second branch 14 that is situated downstream from the bypass device 40 is provided with a visual indicator 70 (see FIG. 1) that shows whether the fluid has flowed in the downstream portion of the second branch 14, i.e. whether the fluid has passed through the bypass device 40. The visual indicator 70 reveals such passage of fluid when the bypass device 40 is inspected visually. The visual indicator 70 may, for example, be a pop-up type indicator, i.e. it has a tongue that is initially contained inside the visual indicator 70 (the tongue is therefore not visible from the outside). Such a visual indicator 70 is known in the prior art and its operation is not described in detail herein. When the fluid passes through the portion of the second branch 14 that is situated downstream from the bypass device 40 and where the visual indicator 70 is located, the visual indicator 70 causes the tongue to move so that it can be seen by the maintenance crew. Thus on looking at the bypass device 40 (and its visual indicator 70), the crew can tell immediately whether the filter 20 has become clogged and whether or not it has been bypassed.
That clogging detection system associated with the filter 20 nevertheless presents the drawback that it is necessary to inspect the visual indicator 70 in order to know whether the filter has become completely clogged or has been clogged only partially.